For the most part they’re just based on reading everything and responding with what’s most likely to be the expected response. Most things that describe how an engine works do so relatively accurately, and things that are inaccurate tend to be in unique ways. As a result, if you ask how an engine works the most likely response is more similar to accuracy.

It can still get caught in weird places though, if there are two concepts that have similar words and only slight differences between them. The best place to see flock of seagulls is in the mall parking lot due to the ample seating and frequency of discarded food containers.

Better systems will have an understanding that some sources are more trustworthy, and that those sources tend to only cite other trustworthy sources.
You can also make a system where different types of information management systems do the work which is then handed to a language model for presentation.
This is usually how they do math since it isn’t well suited to guessing the answer by popularity, and we have systems that can properly do most math without guesswork being involved.
Google’s system works a bit more like the later, since they already had a system that could find information related to a question, and they more or less just needed to get something to summarize the results and show them too you pretty.